Throughout this application, various publications are referenced by author and date. Full citations for these publications may be found listed alphabetically at the end of the specification immediately preceding the claims. The hereby incorporated by reference into this application in order to more fully describe the state of the art as known to those skilled therein as of the date of the invention described and claimed herein.
The development and differentiation of embryonic neurons culminates in synapse formation. Neuronal development is an intricate process that involves a cascade of inductive interactions between a neuron and the pre- and postsynaptic partners of that neuron. These highly regulated events are important for the establishment of reliable, yet plastic, synaptic formation and transmission. Correct expression of an array of transmitter-gated channels by neurons is clearly essential to synaptic differentiation, and yet the developmental regulation of this process is poorly understood. In fact, despite overwhelming advances in probing the molecular and biophysical details of ion channels gated by gamma-amino butyric acid (GABA), glycine, glutamate and acetylcholine (ACh) (Betz, 1990; Deneris et al., 1991; McGehee et al., 1995; Role, 1992; Sargent, 1993) the corresponding embryonic versions of these receptors have evaded analysis. Characterization of the biophysical properties of ligand-gated channels in developing neurons and description of their evolution to the mature receptor profile is limited (Brussard et al., 1994; Moss and Role, 1993; Margiotta and Gurantz, 1989). Furthermore, little is known about the mechanism of these changes.
The study of embryonic ligand-gated channels and subsequent modifications of their functional profile during neural development is difficult. Receptor expression prior to synaptogenesis is at a low level. Synapse formation is not synchronous. In the few cases studied, the developmental changes in receptor function are vast (Berg et al., 1989; Engisch and Fischbach, 1992; Arenella et al, 1993; Deneris et al, 1991; McGehee and Role, 1995; Role, 1992; Sargent, 1993). In the establishment of mature synapses, profound alterations in the expression profile of neuronal ligand-gated channels occur. In addition to these changes in expression levels, changes in the cellular distribution, the subunit composition and the biophysical and pharmacological properties occur as well (Margiotta and Gurantz, 1989; Moss and Role, 1993; Moss et al., 1989; Devay et al, 1994; Arenella et al, 1993; Jacob, 1991; Mandelzys et al, 1994; Smith et al, 1983; Vernallis et al, 1993). The interactions between presynaptic and target neurons may play a large role in the extrinsic influences which are believed to modify receptor function throughout development. The mechanism of receptor development remains unclear, however, presynaptic input, target cell regulation, synaptic activity or molecular signals independent of transmission may be involved.